Small and wide-angle imaging optical systems are being desired for onboard applications and monitoring applications. However, in an optical system which only has aspheric lenses, the number of lenses becomes as many as 5 or more, thus inducing a problem of elongated optical length as well as a problem of material cost. As a method of reducing the number of lenses, lens constructions using diffraction gratings have been proposed (see Patent Documents 1, 2, and 3). A diffraction grating has inverse dispersion and anomalous dispersion, and has a considerable chromatic aberration correction ability.
Patent Document 1 proposes a triple-lens construction utilizing a chromatic aberration correction ability of a diffraction grating(s). Further reduced in number therefrom, Patent Document 2 proposes a double wide angle imaging lens construction. Patent Document 3 proposes a lens such that a ratio between the focal length of the optical system and the focal length of a diffraction grating is adjusted so as to correct a chromatic aberration.